Hi Laura,
this means..
The controller session tables were not fully purged after a controller fast failover event, the system needed multiple purges to re-synchronize the table. The log message will show up as an event, but the system would eventually synchronize and the MU sessions would be complete, no loss to controller or user functionality.
Maybe increase the fast failover time?!
So if the message happens very often I would upgrade the Controller to the newest code.
Regards
Umut Aydin